Above-room-temperature strong intrinsic ferromagnetism in 2D van der Waals Fe3GaTe2 with large perpendicular magnetic anisotropy

The absence of two-dimensional (2D) van der Waals (vdW) ferromagnetic crystals with both above-room-temperature strong intrinsic ferromagnetism and large perpendicular magnetic anisotropy (PMA) severely hinders practical applications of 2D vdW crystals in next-generation low-power magnetoelectronic and spintronic devices. Here, we report a vdW intrinsic ferromagnetic crystal Fe3GaTe2 that exhibits record-high above-room-temperature Curie temperature (Tc, ~350-380 K) for known 2D vdW intrinsic ferromagnets, high saturation magnetic moment (40.11 emu/g), large PMA energy density (~4.79 × 105 J/m3), and large anomalous Hall angle (3%) at room temperature. Such large room-temperature PMA is better than conventional widely-used ferromagnetic films like CoFeB, and one order of magnitude larger than known 2D vdW intrinsic ferromagnets. Room-temperature thickness and angle-dependent anomalous Hall devices and direct magnetic domains imaging based on Fe3GaTe2 nanosheet have been realized. This work provides an avenue for room-temperature 2D ferromagnetism, electrical control of 2D ferromagnetism and promote the practical applications of 2D-vdW-integrated spintronic devices.

Minor concerns 1. How can be the p-type conductivity defined in line 171 through Fig. S8a~d? 2. In imaging magnetic domain structure by MFM, the roughness of SiO2/Si substrate can affect to the evolution of the single domain structure at thinner crystal. It's better to transfer Fe3GaTe2 crystal to thick hexagonal-Boron nitride (h-BN) to get exact thickness needed for evolution of single domain structure. 3. The range of electric field for gating used for controlling the domain wall motion is too narrow. To gain a better insight of the gating effect, further increase or decrease of electric field seems to be needed.
vdW magnetic materials are of high interest in condensed matter and material physics. Fe3GaTe2 has the highest curie temperature among them, and will be intensively researched. However, the experimental data and interpretation of it in 9th and 10th paragraph seems to be mis-approached and need more consideration to give better interpretation. I think paragraphs mentioned above is not adequate for the publication, so it is recommended to publish without them.
Reviewer #2: Remarks to the Author: Essentially, I think this paper should be accepted for publication in nature communications, because this contains very new and interesting idea (proposal) and experimental results of Fe3GaTe2 which possesses 2D layered structure with relatively high Curie temperature just beyond room temperature, leading to some applications as multifunctional spintronics devices. Device fabrications are well designed and their characterizations are highly sophisticated. I believe the technical level of this paper is extremely high and reliable. Such a propose of nano-device is very interesting: especially anomalously angle-dependent Hall devices, and gate-tunable ferromagnetic domain evolutions due to its 2D ferromagnetism. However, they should refer 2 original papers which first discovered this compounds (Fe3GaTe2) as room-temperature 2D ferromagnet for the first time (1) and synthesized its single crystal and discovered a large magnetic anisotropy in Fe3GaTe2 (2) (2013) 124711. If they refer these papers, the present paper will be much better and more valuable with much reliability. They did nicely exhibit the experimental data of this material as magnetic nano-device by utilizing reliable experimental techniques and succeeded in showing interesting experiments in this field. Therefore, I recommend this paper should be accepted for publication after this modification.
The manuscript describes magnetic characteristics of vdW magnetic material, Fe3GaTe2. The authors synthesized Fe3GaTe2 for the first time via self-flux method and characterize their magnetic property from bulk to nanoscale-thickness crystal. They also measured the control of the magnetic domain wall motion by electrical gating.
They attribute the controllability of domain wall motion to resultant surface charge accumulation. vdW magnet materials have received great interest due to their exotic magnetic properties, and the material the authors synthesized is the ferromagnetic material with the highest intrinsic curie temperature reported until now. As its curie temperature is higher than room temperature, which is crucial for application and commercialization, this paper is adequate to be published in Nature Communications.
However, before publication, the authors need to have clear explanation for several major, and minor concerns.

Reply:
Thanks reviewer for recognizing the importance and novelty of our work. manuscript. Please note that the internal phase angle difference of 18, 20, 24, 28 nm nanosheets are negligible, suggesting the single-domain structure. Also, the internal phase angle difference is only ~0.019° even in the typical "inhomogeneous area" of the 16 nm nanosheet, which is much smaller (~6-20.5 times) than thicker nanosheets (>28 nm) with typical multi-domain structure. Therefore, we consider that the internal phase angle difference of 16-28 nm Fe3GaTe2 thin nanosheets are all negligible, and thus, corresponding the single-domain structure. The specific analysis is as follows: the internal phase angle difference of 28 and 18 nm nanosheets are negligible (Fig. R1i, j for reply only), corresponding the typical single-domain structure confirmed by nonstripe MFM images. Besides, we also carefully measured a typical "inhomogeneous area" (see green circle in MFM image in Fig. R1c for reply only) inside the 16 nm nanosheet, and the phase angle difference is only ~0.019° (Fig. R1k for reply only), ~6-20.5 times smaller than internal phase angle difference of multi-domain nanosheets (as mentioned above, ~0.219°, ~0.114°, ~0.39° and ~0.234° for 94, 69, 47, 38 nm nanosheets, respectively), so we think that the 16 nm nanosheet can be considered as uniform as single domain confirmed by non-stripe MFM images. Reply: We agree with your statement. In this work, the gate-tunable magnetic domain evolution should be understood by the domain redistribution. Therefore, in this work, our statement that domain wall motion 'controlled' by electric field is not accurate, and control of domain population or domain redistribution by electric field may be more appropriate.
4. In Fig. 4g and Fig, 4h, as gating electric field increases, phase angle was red-shifted.
They observed the phenomena and concluded that it is caused by the change of magnetic anisotropy energy by the surface charge accumulation. They used thick Fe3GaTe2 nanosheets whose DOS at fermi level is very large, which means the system is hardly influenced by external field. Using thinner Fe3GaTe2 may be better to observe the phenomena.

Reply:
As reviewer note, the van der Waals ferromagnetic Fe3GaTe2 crystal reported in this work is a metallic material, and its DOS at Fermi level is large, which means the system is hardly influenced by external electric field. Also, although the monolayer Fe3GaTe2 crystal shows smaller total DOS than that of bulk and trilayer Fe3GaTe2 crystals (see Fig. S13b), its DOS at Fermi level is still not too small, suggesting the finite effect of electric-field regulation even in monolayer crystal. Furthermore, for tests, we need nanosheet with some area, but much thinner nanosheets have too small areas to be tested. Therefore, after careful evaluation and discussion, we have decided to remove the content about electric-field-regulated domain changes, as suggested by reviewer in following comments, and we may focus on this issue in next work.

From line 241 to 243, they mentioned expansion or contraction of domain wall.
Basically, domain wall is boundary between domains and hardly expanded or contracted if the magnetic anisotropy is strong. Fe3GaTe has very strong magnetic anisotropy in z-axis. I cautiously assume authors mis-stated the word domain wall.
Better to revise statements.

Reply:
We acknowledge that it is a mistake in our original manuscript. Domain walls are the transition regions between adjacent domains where the spontaneous magnetization gradually changes from one direction to another (Rep. Prog. Phys. 24 116, 1961). It is hard to make the domain walls expand or contract as the strong perpendicular magnetic anisotropy in Fe3GaTe2 crystal. In fact, what we want to express is that the electric field can control domain evolution rather than domain wall evolution. Therefore, in this work, we acknowledge that the description of domain wall expansion and contraction is inappropriate. As we mentioned in Major concerns 3, control of domain population or domain redistribution by electric field may be more appropriate. As we mention above, we remove this content and may focus on this issue in next work.

Reply:
In this work, the roughness of the commercial SiO2/Si substrate we used in the MFM test is below 0.5 nm (Suzhou Research Materials Microtech Co., Ltd). Considering the thinnest nanosheet measured by MFM test in this work is a 16 nm Fe3GaTe2 nanosheet rather than an atomically-thin sample, we consider the effect of SiO2/Si substrate's roughness on magnetic domain evolution is negligible. Also, please note that the temperature and electric-field tuned magnetic domain evolution of monolayer diluted ferromagnetic crystal can also present on SiO2/Si substrate directly (Adv. Sci. 1903076, 2020).
3. The range of electric field for gating used for controlling the domain wall motion is too narrow. To gain a better insight of the gating effect, further increase or decrease of electric field seems to be needed.

Reply:
We agree with your statement. Further increase or decrease of electric field is expected to further influence the magnetic domain in ferromagnetic Fe3GaTe2 crystal. As we state in Major concerns 4, van der Waals ferromagnetic Fe3GaTe2 crystal reported in this work is a metallic material, and its DOS at Fermi level is large, which means the system is hardly influenced by external electric field. Even if it can be tuned slightly in the larger gate voltage, the results may still unsatisfactory, and the practical applications are limited due to more power consumption. Therefore, after careful evaluation and discussion, as you suggest, we have decided to remove the content about electric-fieldregulated domain changes.
vdW magnetic materials are of high interest in condensed matter and material physics.
Fe3GaTe2 has the highest curie temperature among them, and will be intensively researched. However, the experimental data and interpretation of it in 9th and 10th paragraph seems to be mis-approached and need more consideration to give better interpretation. I think paragraphs mentioned above is not adequate for the publication, so it is recommended to publish without them.

Reply:
Thanks again for the reviewer's affirmation and important suggestions on this work.
After careful evaluation and discussion, we have decided to accept your comments.
Specifically, we decide to remove the content about electric-field-regulated domain changes, that is, Fig. 4e-h. We retain other direct room-temperature magnetic domain imaging data because we believe that the presentation of magnetic domain structure is important and has reference value for the further study of 2D ferromagnetism in Fe3GaTe2. At least, in this work, we have successfully achieved direct magnetic domain imaging at room temperature in Fe3GaTe2 ferromagnetic 2D crystals. This result provides additional persuasive evidence of intrinsic room-temperature strong ferromagnetism in Fe3GaTe2 nanosheet. Anyway, following your advice, we have removed some inappropriate statements, especially about electrical-field gate-tunable content. We have modified Reviewer #2 (Remarks to the Author): Essentially, I think this paper should be accepted for publication in nature communications, because this contains very new and interesting idea (proposal) and experimental results of Fe3GaTe2 which possesses 2D layered structure with relatively high Curie temperature just beyond room temperature, leading to some applications as multifunctional spintronics devices. Device fabrications are well designed and their characterizations are highly sophisticated. I believe the technical level of this paper is extremely high and reliable. Such a propose of nano-device is very interesting: especially anomalously angle-dependent Hall devices, and gate-tunable ferromagnetic domain evolutions due to its 2D ferromagnetism. However, they should refer 2 original papers which first discovered this compounds (Fe3GaTe2) as room-temperature 2D ferromagnet for the first time (1) and synthesized its single crystal and discovered a large magnetic anisotropy in Fe3GaTe2 (2) they refer these papers, the present paper will be much better and more valuable with much reliability. They did nicely exhibit the experimental data of this material as magnetic nano-device by utilizing reliable experimental techniques and succeeded in showing interesting experiments in this field. Therefore, I recommend this paper should be accepted for publication after this modification.

Reply：
Thank the reviewers for their recognition of our work and suggestions for improving the references. Please note that these two original papers mentioned by reviewer are about Fe-Ge-Te system rather than our Fe3GaTe2. But they are still important for 2D ferromagnetism and we have referred them in our revision now (labeled ref. 14, 15 in revised manuscript).